Hsp90 is a protein chaperone that acts to stabilize and/or
activate a number of proteins required for cellular signaling
pathways, including several known determinants of carcinogenesis
such as HER2 (Erb2), B-Raf, c-Kit, Flt-3 and Raf-1. Hsp90 is
effectively inhibited by benzoquinone ansamycins, such as
geldanacmycin and its derivatives, which bind to the ATP binding
site in the N-terminal domain. Kosan's lead Hsp90 inhibitor,
tanespimycin (17-AAG), is a geldanamycin derivative. Kosan's
third-generation Hsp90 inhibitors are non-benzoquinone ansamycins
that have been genetically engineered to retain a high degree of
specificity of tanespimycin for the ATP binding pocket of Hsp90 yet
do not rely upon activation of the quinone moiety for maximal
inhibitor activity. These third-generation Hsp90 inhibitors are
highly potent molecules in tumor models with properties that
significantly distinguish them from other Hsp90 inhibitors.

Data on Kosan's third-generation Hsp90 inhibitors were presented
in two posters at the 2008 Annual Meeting of the American
Association for Cancer Research (AACR), being held in San Diego,
CA.

Abstract 5636, "Correlation between In Vitro and In Vivo
Activities of Hsp90 Inhibitors," presented data showing the
KOS-2484, Kosan's lead non-benzoquinone ansamycin, has demonstrated
the tightest binding affinity for Hsp90 reported to date,
accumulates in tumor cells and has superior retention properties
leading to a long lifetime in tumor tissue. These properties of
KOS-2484 result in this molecule showing pronounced efficacy in
multiple preclinical tumor models.

Kosan Biosciences is a biotechnology company advancing two new
classes of anticancer agents through clinical development -- a
Hsp90 (heat shock protein 90) inhibitor and an epothilone. Hsp90
inhibitors have a novel mechanism of action targeting multiple
pathways involved in cancer cell growth and survival. Tanespimycin
(KOS-953) is being tested in combination with Velcade(R)
(bortezomib) in patients with multiple myeloma in a clinical
program called TIME. Tanespimycin is also being studied in
HER2-positive metastatic breast cancer in combination with
Herceptin(R) (trastuzumab). Epothilones inhibit cell division with
a mechanism of action similar to taxanes, one of the most
successful classes of anti-tumor agents. KOS-1584 is in Phase 1
clinical trials in patients with solid tumors. Kosan's motilin
agonist compound, KOS-2187, licensed to Pfizer for development in
gastroesophagel reflux disease, is in a Phase 1 trial.

This press release contains forward-looking statements within
the meaning of the "safe harbor" provisions of the Private
Securities Litigation Reform Act of 1995 (the "Act"). Such
forward-looking statements include but are not limited to
statements regarding the further development and potential safety,
efficacy, regulatory status, commercial potential and other
characteristics of Kosan's product candidates; the continuation of
current clinical trials; the initiation of additional clinical
trials and the timing thereof and the use of Kosan's financial
resources. Words such as "will," "expect," "believe," "may,"
"intend," "plan," "potential" and similar expressions are intended
to identify forward-looking statements. Any statements contained in
this press release that are not statements of historical fact may
be deemed to be forward-looking statements. These forward-looking
statements are based upon Kosan's current expectations.
Forward-looking statements involve risks and uncertainties. Kosan's
actual results and the timing of events could differ materially
from those anticipated in such forward-looking statements as a
result of these risks and uncertainties, which include, without
limitation, risks related to the uncertain progress and results of
Kosan's preclinical and clinical testing, including the risks that
studies and trials may not demonstrate safety and efficacy
sufficient to initiate clinical trials on the timing currently
anticipated, or at all, continue clinical development, obtain the
requisite regulatory approvals or result in a marketable product;
the conduct of clinical trials; manufacturing; regulatory approval
requirements and process; the effort and expense necessary for
further development of Kosan's product candidates, including the
costs of bortezomib; intellectual property matters, including
Kosan's ability to obtain valid and enforceable patents covering
its product candidates; Kosan's dependence on its collaboration
with Pfizer for development of its motilin agonist product
candidate; Kosan's need for additional financing and Kosan's
strategy to enter into partnering or licensing arrangements. These
and other risk factors are discussed under "Risk Factors" in
Kosan's Annual Report on Form 10-K for the year ended December 31,
2007 and other periodic filings with the SEC. Kosan expressly
disclaims any obligation or undertaking to release publicly any
updates or revisions to any forward-looking statements contained
herein.

Velcade(R) (bortezomib) is a registered trademark of Millennium
Pharmaceuticals, Inc.

Herceptin(R) (trastuzumab) is a registered trademark of
Genentech, Inc.

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